1. Field of the Invention
The present invention relates to image recording apparatuses such as a copying machine, a facsimile, and a printer. Particularly, the present invention relates to an image recording apparatus that forms an image by discharging gaseous ink intermittently and having the ink attached to or permeated selectively on a recording medium, characterized in the structure of a head.
2. Description of the Background Art
As an image recording system of the conventional discharge type, an ink jet system, electrostatic recording system, and the like are known. In an ink jet system, the liquid ink stored in a tank is pressurized with a piezoelectric element by an electrical signal corresponding to image data, whereby ink is discharged from a nozzle.
In an electrostatic recording system, powder or fluid (gaseous) ink is charged to be discharged from a nozzle by electrostatic attraction. Printing is carried out by opening/closing a shutter provided at the leading end of the nozzle according to an electrical signal corresponding to image data.
The technique of attaching ink on a recording medium by discharging gaseous ink is known. This technique is advantageous in that clogging in a nozzle from which gas is discharged seldom occurs. Furthermore, the resolution is high and the gradation superior since the pixel can be recorded in a molecular state. A clear print out with almost no blur can be obtained.
FIG. 1 shows a structure of a conventional image recording apparatus employing the technique of discharging gaseous ink to be attached to a recording medium. This is disclosed in Japanese Patent Publication No. 56-2020.
Referring to FIG. 1, an image recording apparatus includes a print head 201 accommodating ink 203 therein, a charger electrode 204 for charging gaseous ink 203A, a power source 205 connected to charger electrode 204 and print head 201 for supplying power to charger electrode 204, electric field lenses 206 and 207 for converging charged gaseous ink 203A, a back electrode 211 for inducing charged gaseous ink 203A, a recording medium 212 on which an image is recorded, a signal source 210 for supplying an image data signal, and an electric field shutter 215 for controlling the amount of ink discharge according to a signal from signal source 210. Print head 201 includes a heater device 202, and ink 203 that is heated by heater device 202 to attain a gaseous phase. Heater device 202 includes an electric heater 213 and a power source.
In operation, ink 203 inside print head 201 is heated and gasified by heater device 202. The gasified ink 203A is jetted out from print head 201 and charged by charger electrode 204 in passing charger electrode 204. The charged gaseous ink 203A is converged by electric field lenses 206 and 207 and sprayed towards back electrode 211 with the amount of discharge controlled by signal source 210 in passing through electric field shutter 215. The discharged ink forms an image on recording medium 212.
The following problems are encountered in the conventional art. In the ink jet system, there was a problem that pressure becomes insufficient due to air introduced into the ink tank, whereby printing is inhibited. There is also a problem that usage of liquid ink causes ink clogging at the nozzle and ink blur on the recording medium to result in degradation of the picture quality.
In the electrostatic recording system, there was a problem that, when powder ink is used, the ink particles coagulate into blocks to cause clogging. When liquid ink is used, the problems similar to those of the ink jet system such as clogging and blurring are encountered.
In the aforementioned method of attaching ink onto a recording medium by discharging gaseous ink as shown in FIG. 1, the gaseous ink will be constantly be discharged. The running cost is increased since ink not used in recording will be wasted. Furthermore, a device for collecting the gaseous ink not used, and a device for cleaning the surroundings of electric field shutter 215 are required. This is disadvantageous from the standpoint of maintenance and down-sizing of the apparatus. The transfer of gaseous ink 203A from print head 201 is set forth in the following. When ink 203 is gasified, the volume thereof is expanded. The pressure in ink head 201 is increased, whereby gaseous ink 203A is jetted out. Thus the response of a printing operation with respect to a print out command is not so good. The printing operation in response to a printing command is also affected by the amount of ink 203 within print head 201 to result in variation in density. This will degrade the print out quality.
In view of the above problems, there is an approach, as related art, to obtain an image by discharging gasified ink intermittently according to an electrical signal corresponding to image data to be recorded to be attached onto or permeated into a recording medium. An apparatus realizing this method includes a heater device for heating ink, a discharge device for discharging ink, and a discharge control device for providing control to discharge ink intermittently according to an electrical signal corresponding to image data to be recorded.
This method can be implemented by either of the following three procedures. In the first procedures ink is charged and then heated to be gasified. The gaseous ink is discharged by means of a back electrode disposed at the backside of a recording medium. In this case, the ink to be charged is powder or liquid.
As a second procedure, ink is heated to be gasified. The heated gas is charged, so that the gas is discharged by means of a back electrode disposed at the backside of a recording medium. The ink subjected to heating is in the form of powder or liquid.
The apparatus employing the first and second procedures includes a heater device for heating ink, a discharge device for discharging ink, and a discharge control device for providing control to discharge ink intermittently according to an electrical signal corresponding to image data to be recorded. The discharge device includes a charger electrode for charging ink, and a back electrode disposed at the backside of a recording medium for inducing the charged ink onto the recording medium. The discharge control device includes a shutter unit for controlling ink discharge physically or electrically, and a control unit for controlling the shutter unit by providing a signal corresponding to an electrical signal input with respect to image data. The heater unit, the charger electrode unit, and the shutter unit integrally form a print head.
In addition to the first and second procedures, there is a third procedure for electrically controlling ink discharge. The discharge hole of ink is provided with a slit configuration. A plurality of electrodes are provided at both ends at the longer side of the slit hole with a width corresponding to the recording pixel.
Referring to FIG. 2, an image recording apparatus according to the above-described second procedure includes a print head 10 with a discharge hole 14, a heater device 2 with an electric heater 13 for heating and gasifying powder ink 3 in print head 10, a charger electrode 4 for charging gasified ink 3B, a back electrode 11 disposed opposite to discharge hole 14 at the backside of recording medium 12 arranged facing discharge hole 14 for including charged gaseous ink 3B to the recording medium side, an electric field shutter 8 with electric field shutters 8A and 8B, provided at the discharge hole 14 portion, controlled to intermittently discharge gasified ink 3B according to an electrical signal corresponding to image data to be recorded, and a control unit 9 for controlling electric field shutters 8A and 8B. Charger electrode 4 is formed of a thin wire electrode of 50-80 .mu.m diameter.
Referring to FIG. 3, electric field shutter 8 includes a plurality of discharge holes 14 for gasified ink 3B. Electric field shutters 8A and 8B are provided at either side of discharge hole 14. The plurality of discharge holes 14 are provided over a length corresponding to the printout width. The interval of the discharge holes is 200 .mu.m with a recording density of 150 dpi. Electric field shutter 8 has one side 8A grounded, and the other provided with electrode 8B in a comb-like manner at the interval of 169 .mu.m corresponding to the recording density.
Referring to FIG. 2 again, ink 3 is heated by heater device 2 to be gasified in a printout operation. Using colored ink, ink coloring materials set forth in the following can be employed. For yellow, anthra isothiazole type, quinopthalone type, pyrazolone type, pyridone azo type, styryl type, and the like can be used. For magenta, anthraquinone type, dicyano imidazole type, thiadiazole azo type, tricyanovinyl type, and the like can be used. For cyan, azo type, anthraquinone type, napthoquinone type, indoaniline type, and the like can be used.
Ink 3 is gasified to result in gaseous ink 3B. By applying a voltage of +2 to 5 kV to charger electrode 4, corona discharge is induced towards grounded heater device 2. The gaseous ink 3B is charged with plus charges. The charged gaseous ink 3B is induced onto the recording medium as a result of applying a voltage of -0.5 to -2 kV to back electrode 11 arranged at the backside of the printing face of recording medium 12. Electric field shutter 8 is controlled to a allow/prevent passage of gaseous ink 3B by having a voltage of 50 V to 1 kV applied to the electrode by an output signal of control unit 9 corresponding to an electrical signal of image data to be recorded. Gaseous ink 3B passing through electric field shutter 8 is induced by the charger electrode 4 and back electrode 11 to be attached on recording medium 12. Thus, printing is carried out.
Although ink 3 prior to a heating process is described as powder ink, fluid ink can be used. Fluid ink is advantageous in that the transportability is superior and the amount of energy required for gasification is reduced.
Referring to FIG. 4, an image recording apparatus of the above-described first procedure includes a print head 10 with a discharge hole 14A, a heater device 2 with an electric heater 13 for heating and gasifying liquid ink 3A in print head 10, a charger electrode 4A formed of two electrodes for charge injection, provided to sandwich the stored ink 3A for charging liquid ink 3A, a back electrode 11 disposed opposing discharge hole 14A at the backside of recording medium 12 arranged facing discharge hole 14 for inducing charged gasified ink 3B towards the recording medium 12 side, electric field shutters 8A and 8B provided at the discharge hole 14A portion, controlled to discharge gasified ink 3B intermittently according to an electrical signal corresponding to image data to be recorded, and a control unit 9 for controlling electric field shutters 8A and 8B.
Referring to FIG. 5, discharge hole 14A of ink 3B has a slit configuration. Electric field shutter 8 is provided at both lower sides of the slit. This slit has a length corresponding to the printing width. It is approximately 200 mm for A4-size and approximately 140 mm for A5-size, for example. The recording density is 150 dpi, and the slit width is 200 .mu.m. A slit-shaped discharge hole has the merit that clogging occurs more scarcely than the discharge hole 14 shown in FIG. 3 provided at an interval according to the resolution. Electric field shutter 8 has one side 8A grounded, and the other supplied with an electrode 8B in comb-like manner at an interval of 169 .mu.m corresponding to the recording density.
Referring to FIG. 4 again, a potential difference of 2 to 5 kV is established across charger electrode 4A to inject charge to ink 3A. As a result, ink 3A is charged with plus charges. By gasifying charged ink 3A by heater device 2, gaseous ink 3B is generated maintaining the charged state. Ink 3B is induced onto a recording medium by back electrode 11 arranged at the backside of the printing face of recording medium 12 by having a voltage of -1 kV applied.
A voltage of generally 50 V to 1 kV is applied to electric field shutter 8, so that passage of gaseous ink 3B is inhibited. By an output signal of control unit 9 corresponding to an electrical signal of image data to be recorded, the potential of electric field shutter 8 corresponding to each pixel is controlled to allow passage of gaseous ink 3B. Gaseous ink 3B passing though electric field shutter 8 is induced by back electrode 11 to be attached on recording medium 12. Thus, printing is carried out.
Although ink 3A prior to a heating process is described as fluid ink, powder ink can also be used. In the case of powder ink, frictional charging, for example, can be used besides the above-described charging method by charge injection. Fluid ink is advantageous over powder ink in that charge variation is small and efficient charging is allowed. Powder ink is characterized in that leakage from print head 1 is low.
The discharge hole of ink 3B may be provided at an interval according to resolution as shown in FIG. 3.
An other example of the electric field shutter unit will be described with reference to FIG. 6. A plurality of discharge holes 14B of gasified ink 3B are provided in an electric field shutter 608. Electric field shutters 608A and 608B are provided above and below each discharge hole 14B. The plurality of discharge holes 14B are provided over a length corresponding to a printout width. For example, it is approximately 200 mm for A4 size and approximately 140 mm for A5 size. The interval of discharge holes 14B is 169 .mu.m with the recording density of 150 dpi. Electric field shutter 608 is formed of a ring-like electrode formed above and below discharge hole 14B. Electric field shutter 608A has one side grounded and electric field shutter 608B of the other side is connected to a power source of high voltage. When a high voltage is applied to electric field shutter 608B formed of an electrode, an electric field is generated from electric field shutter 608B to electric field shutter 608A. Therefore, discharge of the positively charged ink is prevented. Conversely, when no voltage is applied to electrode 608B, no electric field is generated across both electrodes. Therefore, ink is discharged from discharge hole 14B.
According to the above-described first and second procedures of the related art, discharge of unrequired ink is inhibited due to the gaseous ink discharge operation controlled within the print head. It is characterized by its superior printing efficiency.
Since the image recording apparatus includes a print head with the integral heater device, charger electrode unit, and shutter unit according to the first and second procedures, the possibility of ink adhering to the shutter unit to result in clogging can be alleviated. Furthermore, the device for collecting the attaching ink and the cleaning device are no longer required. Particularly in the second procedure, even ink of an insulating material can be charged and is not easily subjected to environmental influence since gasified ink is charged.
In the first procedure, charging is effected without generation of harmful substances such as ozone since solid or liquid ink is charged and then heated to be gasified. The usage of a slit-shaped discharge hole is advantageous in that clogging is prevented and that it is not necessary to provide a discharge hole for each pixel.
The first and second procedures of the related art have issues set forth in the following. When ink is heated, the pressure within the ink head is increased. If the pressure becomes higher than the outside pressure of the head, there is a possibility that ink will leak out from the slit even when there is no electric field by the back electrode. This will be described in detail with reference to FIGS. 2, 7 and 8.
When an image output signal is transmitted to an image recording apparatus from another information equipment or an operator, a head control signal is transmitted to a head 1 from the CPU of the image recording apparatus. A temperature control device 16 is triggered at the rise (point A) of this signal to drive heater device 2. Here, a voltage of +2 to 5 kV is applied to charger electrode 4.
By actuation of heater device 2, solid or liquid ink 3 is heated and then gasified when the vaporization temperature of T.sub.g is exceeded. The amount of gaseous ink 3B increases over time to arrive at a threshold value at point B'. This threshold value refers to the amount of gaseous ink sufficient for discharge and that enables printing when this value is exceeded. This state also implies that head 1 is filled with gaseous ink 3B of high pressure.
Heater device 2 is controlled by temperature control device 16 so as to achieve a certain temperature T higher than ink vaporization temperature T.sub.g. Therefore, T=T.sub.g +.DELTA.T. When ink vaporization temperature T.sub.g is 140.degree. C., T=155.degree. C. with, for example, .DELTA.T=15.degree. C.
Since electric field shutter 8 is turned off from point B' to point D, gaseous ink particles 3B of high pressure will be discharged due to the pressure difference outside and inside the head even if an electrical force is applied by back electrode 11. The force exerted on the ink is caused by the pressure in the head attaining high pressure, and not by the electrical force from back electrode 11. Therefore, ink particle 3B is discharged by the pressure in the head.
Electric field shutter 8 operates to block and discharge the charged ink particles when turned on and off, respectively. The undesired ink leakage of issue occurs only when electric field shutter 8 is turned off. This ink leakage occurs during point B' to point D corresponding to the OFF state of electric field shutter 8.
At a certain time point of D where printing is enabled, a print start/end signal is output to the control unit of discharge control device 9 from the CPU. The control unit is triggered at the rise of the print start/end signal to request transmission of image data from a memory not shown. At completion of data transfer, an on/off signal corresponding to image data is provided to electric field shutter 8. Electric field shutter is connected to a power source not shown supplying a voltage of 500 V. Application of a voltage of 500 V is conducted/not conducted according to image data. During printing, the print start/end signal maintains a high level.
When printing ends, the print start/end signal attains a low level at point E. Triggered at this falling point, heater device 2, charger electrode 4, electric field shutter 8 and back electrode 11 are all turned off.
Even if the application of a current to the heater is suppressed, gaseous ink 3B that was not discharged and remaining in head 1 is not cooled immediately. Therefore, ink discharge will occur not corresponding to image data due to difference in the pressure outside and inside the head.
Thus, this undesired ink leakage may occur at the two stages of before initiation and after completion of printing.